user_namespaces.7: Clarify that rules for writing to map files also apply to gid_map

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/user_namespaces.7

@@ -278,13 +278,17 @@ file of
 of the process in the namespace may be written to
 .I once
 to define the mapping of user IDs in the new user namespace.
-(An attempt to write more than once to a
+An attempt to write more than once to a
 .I uid_map
 file in a user namespace fails with the error
-.BR EPERM .)
+.BR EPERM .
+Similar rules apply for
+.I gid_map
+files.
 
 The lines written to
 .IR uid_map
+.RI ( gid_map )
 must conform to the following rules:
 .IP * 3
 The three fields must be valid numbers,
@@ -305,7 +309,8 @@ and
 .BR pwrite (2)
 can't be used to write to nonzero offsets in the file).
 .IP *
-The range of user IDs specified in each line cannot overlap with the ranges
+The range of user IDs (group IDs)
+specified in each line cannot overlap with the ranges
 in any other lines.
 In the current implementation (Linux 3.8), this requirement is
 satisfied by a simplistic implementation that imposes the further
@@ -313,6 +318,9 @@ requirement that
 the values in both field 1 and field 2 of successive lines must be
 in ascending numerical order.
 .IP *
+The mapped user IDs (group IDs) must in turn have a mapping
+in the parent user namespace.
+.IP *
 At least one line must be written to the file.
 .PP
 Writes that violate the above rules fail with the error